Optically clear adhesive is used to bond optical elements such as display panels, glass plates, touch panels, diffusers, rigid compensators, heaters, and flexible films like polarizers and retarders. Control of the adhesive flow is very critical in the bonding process. “Overflow” of an adhesive will occur when the adhesive flows out the intended coverage area. Subsequently, the cured adhesive will cause mechanical fitting problems in the assembly process. Furthermore, overflow of the adhesive will contaminate adjacent components in the display assembly. As a result, additional cleaning step will be required before curing, which will increase the cycle time of the manufacturing process.
A method called “damming process” is an attempt to solve this problem. This process consists of two steps. In the first step, the adhesive is applied to the edge of the required area by a dispensing nozzle and cured to form a so-called “dam”. In the second step, optically clear adhesive is filled into the area inside the dam. The “damming process” is not satisfying because it requires two steps of dispensing. Furthermore, the boundary of the dam is not easy to control.
Another attempt to solve this problem is to use optically clear tapes, but it needs a special knife to cut off tapes with tailored size to match the display panel, and furthermore, it leads to a lower brightness and contrast degree of the display due to air gaps or bubbles in assembly of display panel in large size.
Therefore, methods called “stencil printing process” and “slit coating process” have been created to solve this problem. “Stencil printing” is a process by which a viscous material is deposited through the aperture openings of a stencil onto a substrate. For the printing process the stencil is aligned to the substrate and then brought in close or in direct contact with the surface of the substrate. An angled blade, called a squeegee, is used to drive the material across the surface of the stencil at a controlled speed and force. “Slit coating” is employed to dispose adhesives accurately and quickly in precision lamination applications involving the lamination of a glass panel onto a display panel in LCD display, or the lamination of a touch sensitive panel onto a display panel in touch-sensitive electronic devices.
Stencil print process and slit coating process, require adhesives having side curing capability, stable thixotropy and low dielectric constant, which are still challenges in the field of optically clear adhesives.
Therefore, there is a need for optically clear adhesive providing improved side curing capability, stable thixotropy and low dielectric constant.